Photolytic oxidation of ammonium chloride as a source of Cl2 in the atmosphere

Chlorine atoms (·Cl) exhibit high reactivity and exert a substantial influence on the atmospheric oxidation capacity and chlorine cycle in the troposphere. The photolysis of molecular chlorine (Cl 2 ) is a crucial source of ·Cl. However, the daytime peaks of Cl 2 concentration cannot be entirely accounted for by the currently known pathways. Here we show that the photolytic oxidation of ammonium chloride (NH 4 Cl), a ubiquitous component of atmospheric aerosol, serves as an important daytime source of Cl 2 . Laboratory experiments demonstrate that oxygen, water vapour, and acidity are indispensable for Cl 2 generation and release, and that Cl 2 production is enhanced in the presence of black carbon aerosol. Field observation combined with model simulation demonstrates that the mechanism explains 12-55% of daytime Cl 2 concentration. These results reveal a photoactivation pathway for chlorine production that depends only on chlorine salts and solar radiation, with significant implications in regions with abundant chloride salts.